Melt 2 tablespoons butter in large skillet over medium heat.Add the sliced mushrooms and sauté until they become soft and release their juices; about 4-5 minutes.
Add shrimp, imitation crab, green onions, garlic, and wine (or chicken broth). Cook for 4 minutes, stirring frequently. Remove skillet from heat.
Using a slotted spoon, remove the seafood mixture to a medium mixing bowl, leaving any remaining cooking liquid in the bottom of the skillet. Don't worry if there isn't any liquid left; but if there is, it will add some great flavor to the sauce.To the seafood mix in the mixing bowl, stir in 1 cup of grated Monterey Jack.Set the seafood mixture aside.
Mornay Verde Cream Sauce
In the same skillet, add 4 tablespoons butter to any remaining cooking liquid,and heat over medium-low until it melts and begins to simmer.Reduce heat to low and sprinkle in ¼ cup flour, whisking constantly as you do. Whisk and cook the butter and flour together for 1-2 minutes.
Whisk in half & half and a half-cup of chicken broth all at once. Stir constantly until the sauce becomes thick enough to coat the back of a spoon.
Remove sauce from heat. Add the Monterey Jack, plus the salt and pepper. Whisk constantly until the cheese melts completely.Once the cheese has completely melted, whisk in the salsa verde.
The sauce will be very thick at this point; about the consistency of very pourable nacho cheese. However, if it seems too thick, add a little more chicken broth or water to thin it out.
Roll & Bake
Preheat oven to 350°F | 177°C.
Grease (or spray with non-stick cooking spray) a 11" x 7" glass baking dish.
To the seafood mixture, add 1 cup of the prepared sauce and mix to thoroughly incorporate. Roll a scant ½ cup of the seafood mixture into each tortilla, and place each roll into the baking dish.
Pour the remaining sauce over the prepared enchiladas. Use the back of a spoon to "frost" the top, making sure to completely cover all the edges and let it drip down into the cracks and creases. Mornay sauce is supposed to be very thick; however, if the sauce has thickened too much while you prep the enchiladas (i.e., if it becomes gloppy or too thick to pour), add an additional ¼ - ½ cup of chicken broth (or water) to the mixture to thin it out before pouring it on the enchiladas.
Bake enchilada casserole in preheated oven for 40 minutes.
Notes
How to Make Seafood Chimichangas
If you would prefer to make chimichangas, it is super easy to do using this enchilada recipe. A chimichanga is basically a deep-fried burrito. You can deep fry if you want to, but oven / air-fryer prep is easier. Use medium-large tortillas, and roll the filling inside the tortillas burrito-style.
Oven Chimichangas
Preheat oven to 350°F | 177°C. Heat some canola or olive oil in a heavy skillet over medium-high heat. Fry the filled tortillas in hot oil for 3-4 minutes on each side, until lightly browned.
Bake uncovered on a parchment-lined baking tray for 25 minutes. Serve topped with prepared Mornay verde cream sauce.
Air-Fryer Chimichangas
Brush each side of rolled tortilla with canola or olive oil. Place rolls in basket seam-side down. Do not crowd.
Air fry at 400°F | 205°C for 4 minutes. Flip over and air fry until lightly browned; about 3-4 minutes more. Served topped with prepared Mornay verde cream sauce.